<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <title>Page Title</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <style>
        body{
            background: black;text-align: center;
        }
        #ca1 {
            background: #ffffff;
        }
    </style>
</head>
<body>
    
</body>
    <canvas id="ca1" width="900" height="800"></canvas>
    <script>
        window.onload = function (){
            let ca = document.getElementById('ca1');
            let gd = ca.getContext('2d');
            
            gd.lineWidth = 50;
            gd.strokeStyle = 'blue'
            gd.strokeRect(200,200,300,300);

        }
    </script>
</html>